nonphotosynthetic prokaryotes and consist of a dimer of the Fo b subunits (discussed in

the following section) and a single Fl 8 subunit. The 6 subunit displays a very low level

of conservation across various species. It is a globular protein encoded by the uncH gene

that consists of 177 residues with a molecular weight of 19,332 Da. It plays essential

roles in both the binding of Fl to Fo as well as coupling of the catalytic activities of Fl

and Fo (139, 175-180). Circular dichroism (CD) spectroscopy and sedimentation analysis

studies performed on the 6 subunit suggested a highly helical and elongated conformation

(139).

 Structure of the 8 subunit. A partial high-resolution structure of the 6 subunit has

been solved by NMR (137). During the purification procedure, a truncated form of the 6

subunit was produced by a bacterial protease. It was revealed to be the amino-terminal

134 amino acids (81-134) by mass spectroscopy and N-terminal sequencing. The same

sized 6 subunit fragment was often seen in Fl preparations and could be produced in

isolated E. coli Fl by treatment with trypsin without liberating the 61-134 frOm the Fl

complex (89). Furthermore, purified 81-134 Stably binds to 6-free Fl preparations. The

high affinity of the 61-134 Subunit for Fl indicated that the conformation of the 6 fragment

was preserved during the purification procedure. NMR was performed on both the 61-134

fragment and the intact 8 subunit; however, the quality of data for the intact subunit was

not sufficient enough for structural analysis due to its propensity to aggregate at high

concentrations. To date, the carboxyl-terminal 43 amino acid residues (6135-177) Of the 6

subunit is the only portion of the Fl sector not known at the atomic level.

 The amino-terminal 105 residues of the 6 subunit formed a dense globular domain,

while the region from residues 106-134 was mostly disordered with the exception of one